1. Field of the Invention
This invention relates to an underwater pipe laying apparatus and, more particularly, to a system for displaying the profile of a pipeline as it is laid on the sea floor by a surface vessel.
2. Prior Art Relating to the Disclosure
During the construction of an offshore pipeline the pipeline is normally held on a surface vessel, such as a ship or barge, by a machine called a pipe tensioner whose function is to exert a predetermined and relatively constant tension on the pipeline regardless of the motion of the vessel due to sea movement, wind or forward propulsion, in order to prevent the pipeline from being damaged.
During the pipelaying operation, the pipeline generally extends from the stern of the vessel onto a supporting structure called a stinger. The stinger is sloped toward the seabed to provide a support for the pipeline to follow as the pipeline enters the water. From the end of the stinger to the seabed the pipeline follows a modified catenary profile. The exact shape of the profile depends on the pipe stiffness, the depth of the water, the pipeline tension and any underwater currents. Therefore, the pipeline profile from the vessel to the touchdown point on the seabed is unknown.
The characteristic problem of underwater pipe laying operations is that if the pipeline is played out too rapidly, the curved portions of the catenary may become excessively sharp, causing the pipeline to buckle. Although this problem can be alleviated to some degree by using the load of the pipeline on the pipe tensioner as a measure of pipeline tension and keeping this load at a predetermined value, this does not compensate for the friction of the pipeline supporting mechanisms downstream of the pipe tensioner such as on the stinger as well as environmental conditions external to the vessel. By examining the side profile of a pipeline extending between the vessel and the seabed, excessive curves which are likely to cause buckling may be observed and the tension of the pipeline may be accordingly increased to reduce the curvature.
A similar problem may arise with respect to lateral deflections of the pipelaying vessel, since side buckling may occur if the vessel deviates sufficiently from a desired pipeline route. Furthermore, even where the lateral displacement of the vessel is not sufficient to cause side buckling, the pipeline may be unduly lengthened by following a curved path thereby increasing the cost of the pipeline. By examining the top profile of the pipeline, the lateral position and pointing angle of the vessel may be adjusted to prevent side buckling and ensure a straight pipeline path.
As the pipeline passes over the stinger the coating of the pipeline may be damaged, thereby weakening the corrosion protection of the pipeline at localized areas. By examing the top and side profiles of the pipeline these points of potential pipe failure can be determined and corrective measures taken to avoid pipe rupture.
Another disadvantage of conventional pipe laying systems is that since they do not accurately control the position of the pipeline between the vessel and seabed, they require an unduly wide right of way, even when the position of the vessel can be accurately determined by navigational aids.